Since the advent of inductively coupled plasma (ICP) deep reactive etching (DRIE) technology, various MEMS (micro-electromechanical systems) structures have been proposed comprising an element which is movable laterally in-plane, i.e. in the plane of the semiconductor substrate(s) from which the devices are typically produced. The laterally movable portion may take the form of a laterally deflectable cantilever. This kind of MEMS device benefits from advanced bulk-micro-machining fabrication capabilities of DRIE to form high aspect ratio micro-structures. In some applications of such MEMS devices, the motion of the movable portion is sensed (typically, by capacitive sensing); in other applications, the movable portion is moved by electrostatic actuation.
Unlike surface micro-machining fabrication, DRIE-based fabrication enjoys sticking-free release. In addition, in-plane lateral configuration can be exploited for other advanced features such as elimination of squeeze-film damping effects, comb-drive actuation and over-range self-caging, etc.
The fabrication technologies for in-plane lateral MEMS structures can be classified into two categories. The first category is SOI (silicon-on-insulator) wafer based technology [1, 2], in which a wafer includes a silicon layer formed over an insulating layer. The other category is based on the use of a single homogenous wafer such as crystalline silicon (such techniques are referred to here as “single-wafer-based”). Compared with SOI-wafer based technology, the single-wafer-based technique has advantages in terms of low-cost, simplification and integration compatibility with circuits, etc.
A single-wafer-based technology called “SCREAM” was developed for one-mask or two-mask processes [3, 4]. In this technology, a trench is formed by DRIE and lateral etching is performed by dry isotropic release at the trench bottom, to form lateral MEMS structures. Following this scheme, enhanced isolation capability was realised by chemical vapor deposition of insulating “bus-bars” at the trench bottom [5].
Instead of dry etching release methods, it is known to perform lateral wet release by anisotropic etching of a (111) silicon wafer for the formation of single-wafer-based in-plane lateral MEMS [6, 7]. These above mentioned technologies have been mainly used for capacitive sensors and electrostatic actuators.
Note that the MEMS devices described above have not made use of piezoresistive devices. Although piezoresistive devices have been widely adopted in MEMS devices having a conventional vertical configuration and which are fabricated using conventional anisotropic wet etching and sandwiched bonding, etc., piezoresistive sensing has seldom been employed for MEMS devices with a laterally movable portion. This is because there is no straightforward processing technology available for integration of both piezoresistive sensors and electrostatic actuators on the trench-sidewalls in MEMS devices with laterally movable portions. It is also a challenging task to form electric lead-out connections from the sidewalls to the wafer surface sufficiently precisely and ensure that piezoresistive sensing and electrostatic actuating elements on the trench sidewalls are electrically isolated from each other.
For these reasons, those few MEMS devices having laterally movable portions which use piezoresistive sensors usually locate the piezoresistors on the top surface of laterally movable portion [8]. In this configuration the piezoresistors do not sense the stress at the sidewalls of the movable portion, where stress may be maximal, thereby resulting in low piezoresistive sensitivity.
It is true that an oblique ion implantation method has been proposed for the integration of piezoresistors on the trench-sidewall of lateral MEMS structure [9], but this oblique implantation is not a standard process. It requires that the wafer is inclined at a certain angle in the processing equipment during the implantation. For a cantilever structure, the oblique implantation would have to be processed twice (in order that piezoresistors are formed on both sides of the cantilever). For a square shaped structure, the oblique implementation would have to be performed 4 times. For a circular MEMS structure, uniform sidewall doping cannot be obtained even if the oblique implantation process is performed an unlimited number of times.
One particular difficulty in providing MEMS devices with piezoresistive sensors and electrostatic actuators is the creation of paths for electrical transfer between the wafer surface (where circuitry is located) and the side walls of the movable portion or its surrounding frame, while simultaneously providing adequate electrical isolation between adjacent elements formed on the side walls.